This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the aligned magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the Larmor equation .omega.=.gamma.B in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.o plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillation at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of the B.sub.o field by means of RF pulses through a coil connected to a radio-frequency-transmitting apparatus. Under the influence of RF excitation, magnetization M rotates about the direction of the B.sub.1 field. In NMR studies, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.o field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane precess around the direction of the static field. The vector sum of the spins forms a precessing bulk magnetization which can be sensed by an RF coil. The signals sensed by the RF coil, termed NMR signals, are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the signals. This information is later used to reconstruct images of the object studied in a manner well-known to those skilled in the art.
In performing whole-body NMR studies, it has been found advantageous to increase the strength of the homogeneous magnetic field B.sub.o. This is desirable in the case of proton imaging to improve the signal-to-noise ratio of the NMR signals. In the case of spectroscopy, however, this is a necessity, since some of the chemical species studied (e.g., phosphorus and carbon) are relatively scarce in the body, so that a high magnetic field is necessary in order to detect usable signals. As is evident from the Larmor equation, the increase in magnetic field B is accompanied by a corresponding increase in frequency and, hence, in the required resonant frequency of the transmitter and receiver coils. This complicates the design of RF coils which are large enough to accommodate the human body. One source of difficulty is that he RF field generated by the coil must be homogeneous over the body region to be studied to produce more uniform measurements and images. The production of uniform RF magnetic fields over large volumes becomes increasingly difficult at high frequencies owing to unwanted effects of stray capacitances between different parts of RF coils and between RF coils and surrounding objects or the NMR sample, itself, which limit the highest frequency at which the coil can be made to resonate.
Rf coils which produce substantially homogeneous fields at high frequencies throughout large volumes have been designed. Such coils are disclosed, for example, in U.S. Pat. No. 4,680,548 for use in whole body imaging of hydrogen nuclei at 1.5 Tesla, and U.S. Pat. No. 4,799,016 for use as a local coil in imaging both hydrogen and phosphorous nuclei at 1.5 Tesla. While such coils are capable of producing a homogeneous RF field within their central region of interest when no subject is present, they do not produce a homogeneous RF field when a typical subject is located in the region of interest.
The lack of RF field homogeneity in the subject is due to the fact that the subject has conductivity (.sigma.) and permittivity (.epsilon.) which are much greater than that of air (.sigma..sub.o =0, .epsilon..sub.o =8.854.times.10.sup.-12 farad/m). The effects caused by conductivity (.sigma.) are well understood as explained in "Comparison of Linear and Circular Polarization for Magnetic Resonance Imaging" by G. H. Glover et al. in Journal of Magnetic Resonance 64, 255-270 (1985). Due to this conductivity, eddy currents are induced in the subject by the applied RF magnetic field, and these eddy currents in turn produce an RF magnetic field which adds to that produced by the RF coil. The result is a nonhomogeneous RF field in which the field strength varies as a function of distance around the central axis. As a result, images are produced in which bright areas appear in two quadrants and dark areas appear in the other two quadrants. The well-known solution to this problem is to employ quadrature excitation and reception which rotates the B.sub.1, RF field in the transverse plane to even out the nonhomogeneities due to eddy currents.
Prior RF coil designs also produce a nonhomogeneous RF field in the subject as a result of the high permittivity (.epsilon.) of the subject relative to air. The wavelength of the RF field is shortened in the subject and this produces a standing wave in which the RF field strength varies as a function of radial distance. At 1.5 Tesla, for example, the standing wave produced in the torso of a human subject peaks along the central axis of the imager to produce a bright area in the reconstructed image. At higher field strengths and RF frequencies, the standing wave cycles between peaks and valleys as a function of radial distance from the central axis, and the resulting image has a series of dark and bright rings (axial view) or stripes (sagittal or coronal view). These variations in image brightness can make diagnosis difficult or even impossible in some circumstances.